Conventionally, there has been known a motor-driven, medical or dental contra-angle handpiece. The handpiece has an external cylindrical grip portion and a drive source detachably connected to the proximal end of the grip portion. The distal end of the grip portion has a head in which a treatment or cutting tool is releasably attached. The grip portion is contra-angled so that distal and proximal central axes thereof cross with each other at a certain small angle as shown in JP 2005-520580 A, JP 2009-28512 A, JP 7-51136 B, and JP 2005-42883 A, for example.
The handpieces disclosed in JP 2005-520580 A and JP 7-51136 B are three-stage rotational speed increasing handpiece. According to this handpiece, the rotation from the drive source is transmitted to the first rotation shaft within the external cylinder. The rotation is increased by a pair of engaging gears and transmitted to the second rotation shaft. Further, the rotation is increased by another pair of gears and then transmitted to the third rotation shaft. Furthermore, the rotation is increased and then transmitted to the head potion for driving the treatment tool.
The handpiece disclosed in JP 2009-28512 A is a two-stage rotational speed increasing handpiece. According to this handpiece, the rotation of drive source is transmitted to the first rotation shaft within the external cylinder. The rotation is further increased by a pair of gears and then transmitted to the second rotation shaft. The increased rotation of the second rotation shaft is transmitted to the treatment tool in the head.
The handpiece disclosed in JP 2005-42883 A is a two-stage speed increasing handpiece in which a rotation of the drive source is firstly transmitted to the rotation shaft within the external housing. The rotation of the first rotation shaft is transmitted and increased through a pair of gears. The increased rotation of the second rotation shaft is further increased and then transmitted to the cutting tool.
Another rotation transmission mechanism with a planetary gear train for transmitting the rotation of the drive source to the head is disclosed in JP 2005-42883 A.
Each of the handpieces disclosed in JP 2005-520580 A and JP 7-51136 B employs an internal gear system in which a distal internal gear of the first rotation shaft is engaged with the associated proximal external gear of the second rotation shaft, which ensures an elevated contact or overlap ratio of the gears and, as a result, a smooth engagement thereof to reduce gear noise. It has been known that the contact ratio indicates the number of simultaneously engaging gear pairs in two engaging gears and the elevated contact ratio ensures that the rotational force being transmitted is divided by more gears and, as a result, the rotation is transmitted from one gear to the other smoothly. This in turn reduces wearing of each gear. Therefore, it is preferably that the contact ratio be equal to or more than 1.0, more preferably, equal to or more than 1.2 from a practical point of view.
According to the handpieces disclosed in JP 2005-520580 A and JP 7-51136 B, the second and third shafts are drivingly connected by the external distal and proximal gears thereof, which makes it difficult to provide an elevated contact ratio for the gear system. This means that each engaging tooth bears a larger load or bending force, which fails to attain a stable engagement of the gears. This also increases the noise, wearing, and damages of the tooth.
Also, increasing the diameters of the external gears to increase the contact ratio of the gears results in an unwanted enlargement of the bending portion of the handpiece, accommodating the external gears, which makes it difficult for operators or users of the handpiece to grip it stably or to operate the handpiece as they like. This in turn may result in that the handpiece touches another tooth or teeth of the patient during cutting treatments.
Further, in the medical or dental handpiece disclosed in JP 2009-28512 A, in order to rotate the cutting tool at an elevated high speed, the teeth number of the small external gear is designed to be considerably smaller to that of the large internal gear. This increases a backlash between the gears, which increases gear noise and stress acting on the teeth and decreases durability of the teeth.
The transmission mechanism disclosed in JP 2005-42883 A employs the planetary gear mechanism, which complicates its mechanical structure.
Therefore, it is an object of the present invention to solve those problems and to provide an easy-to-use medical or dental handpiece having an enlarged contact ratio tooth along with an enlarged tooth thickness, ensuring a stable engagement between the gears, an increased durability of the gears, and an elevated rotation number of the cutting tool.